The present invention relates to a device for driving a cross-coil measuring meter used as a vehicle speed meter.
Various measuring meters for visually confirming running conditions are provided on a vehicle. Of those meters, a cross-coil measuring meter is generally used as an analog type of a speed meter for confirming the running speed of the vehicle.
As a general driving device of such a cross-coil measuring meter, a driving device disclosed in Japanese Patent Unexamined Publication No. Hei. 1-201167, for example, as shown in a functional block diagram of FIG. 8, is known.
In this driving device, a running pulse signal the period of which changes in accordance with the speed of a vehicle or the rotational speed of an engine is supplied to a counter circuit 1. In this counter circuit 1, the number of clock pulses of basic clock supplied from an oscillating and dividing circuit 2 to the counter circuit 1 is counted every period of the running pulse, and the counted value is supplied, as digital data, to a sine function generating circuit 3 constituted by a ROM.
The sine function generating circuit 3 supplies a sine duty pulse generating circuit 4 with sine data of an angle .theta. corresponding to the value of digital data from the counter circuit 1. At the same time, on the basis of the relationship sin(90.degree..+-..theta.)=cos .theta., the sine function generating circuit 3 supplies a cosine duty pulse generating circuit 5 with sin(90.degree..+-..theta.) shifted by 90.degree. in phase from the angle .theta. corresponding to the digital data, that is, the data of cos .theta., that is, the cosine data of the angle .theta..
The sine duty pulse generating circuit 4 to which the sine data of the angle .theta. is supplied from the sine function generating circuit 3 generates a duty pulse signal with a fixed frequency on the basis of the basic clock from the oscillating and dividing circuit 2, and outputs the duty pulse signal to a driving circuit 6. In the same manner, the cosine duty pulse generating circuit 5 supplied with the cosine data of the angle .theta. from the sine function generating circuit 3 generates a duty pulse signal with a fixed frequency on the basis of the basic clock from the oscillating and dividing circuit 2, and supplies the duty pulse signal to a driving circuit 7.
Then, the driving circuit 6 supplies a first pulse current I.sub.1 corresponding to the sine data of the angle .theta. from the sine function generating circuit 3, between respective terminals a.sub.1 and a.sub.2 of a first coil L.sub.1 constituting a cross coil L. The driving circuit 7 supplies a second pulse current I.sub.2 corresponding to the cosine data of the angle .theta. from the sine function generating circuit 3, between respective terminals b.sub.1 and b.sub.2 of a second coil L.sub.2 which is disposed perpendicularly to the first coil L.sub.1, and constitutes a cross coil L together with the first coil L.sub.1.
Consequently, a combined magnetic field of a magnetic field generated in the first coil L.sub.1 and a magnetic field generated in the second coil L.sub.2 is generated in the cross coil L. The vector direction of this combined magnetic field is corresponding to the speed of the vehicle or the rotational speed of the engine, that is, a measured value. A magnet rotor Mg in the cross coil L is rotated by this combined magnetic field, so that the vector direction of the combined magnetic field of the cross coil L is pointed by a not-shown pointer fixed to a rotating shaft of the magnet rotor Mg, and the measured value is indicated in cooperation with a not-shown dial plate.
In the above-mentioned cross-coil measuring meter, when the pulse period of the running pulse changes in accordance with the change of the running speed of the vehicle or the rotational speed of the engine, the sine data and the cosine data from the sine function generating circuit 3, hence the first and second pulse currents I.sub.1 and I.sub.2 from the driving circuits 6 and 7, respectively, the combined magnetic field of the cross coil, and the pointing direction of the pointer change following the change of the pulse period of the running pulse.
However, in the cross-coil measuring meter in which the pointer is rotated to a place corresponding to the running speed on the basis of the period of the running pulse signal, once a noise generated at the time of turning on an ignition switch of the vehicle is superimposed on the running pulse signal to generate a pulse of a fractional period in the running pulse signal, the pointer of the speed meter undesirably oscillates notwithstanding the vehicle is at a stop in fact.
As a prior art device to prevent a measuring meter from performing malfunction caused by a pulse signal generated independently of the real running of a vehicle, there is a driving device for preventing a pointer from performing malfunction caused by the influence of a noise generated from chattering of a mechanical switch, for example, as disclosed in Japanese Patent Unexamined Publication No. Hei. 7-294286.
Because of the background that chattering is generated in the structure of a mechanical switch, such a structure is adopted in this driving device that the time of generation of chattering is estimated in advance, and a running pulse signal is masked to prevent it from being reflected in calculation of its period during the estimated time from the leading edge or the trailing edge of the running pulse signal.
However, in the above-mentioned conventional driving device, the time span in which the running pulse signal is masked is fixed to the estimated time of generating chattering of a mechanical switch regarded as a factor of noise generation. Accordingly, if the engine is difficult to start, for example, due to a so-called "fogging" of an ignition plug, noise may be possibly generated for a considerably long time, so that there is a problem that it is not possible to effectively prevent malfunction of a pointer caused by the aforementioned noise at the time of turning an ignition switch on.